The present invention relates generally to magnets, and more particularly to a magnet having a laminated pole piece and having a shim for reducing magnetic field inhomogeneity in the magnet.
Magnets include those having spaced-apart first and second pole pieces with generally opposing first and second pole faces, such as (but not limited to) "C" -shaped magnets. Typically, the pole piece has a laminated portion attached to a non-laminated iron base. The laminated portion includes alternating layers of magnetizable iron and non-magnetizable epoxy. Some magnet applications, such as magnetic resonance imaging (MRI) for medical diagnostics, employ superconducting magnets to generate a high strength magnetic field within an imaging volume located between the pole pieces. The imaging volume must have a low magnetic field inhomogeneity for high quality imaging. Known techniques for reducing the magnetic field inhomogeneity within the imaging volume include using shimming rings attached to each pole face to reduce axisymmetric magnetic field inhomogeneity and using shims adhesively bonded to a shim tray attached to the shimming rings to reduce 3D (three-dimensional) magnetic field inhomogeneity (including non-axisymmetric inhomogeneity and any remaining axisymmetric inhomogeneity). Typically, the laminated portion of the pole piece is configured as a plurality of concentric shimming rings, with a central circular ring surrounded by concentric annular rings, wherein the annular rings are formed from circumferentially-arrayed annular segments which are electrically insulated from each other. The number, mass, and position of the shims on the pole face are determined by shimming techniques known to those skilled in the art. Adhesively bonding a shim to the shim tray is time consuming as the shim must be manually held in place, against the magnetic field, until the adhesive sets. Reshimming the magnet requires debonding the attached shim and cleaning the removed shim and the attachment area on the pole face.
What is needed is a magnet design for a pole face magnet wherein the shims are more quickly and easily installed and removed.